Janus is the ancient Roman god honored by the month named January.
Janus was associated with doors, gates, beginnings, transitions, and endings. He was commonly depicted with two faces, one looking forward and one looking backward. That imagery made him an apt symbolic figure for the passage from one year to the next, although modern New Year's celebrations are not direct replicas of ancient Roman rites.
The Latin name for January is Ianuarius, derived from Janus. In the Roman calendar reforms associated with Numa Pompilius and later Julius Caesar, January became the opening month of the civic year. January 1 was established as the start of the Roman consular year in 153 BCE, helping make the date politically and administratively important.
The Julian calendar reform of 46 BCE further standardized the calendar. The modern Gregorian calendar retained January as the first month, so Janus remains embedded in the language and symbolism of New Year even though Roman religious worship ended long ago.